1. Field of the Invention
This invention relates generally to machines and procedures for printing or reading images--text or graphics--on an image-bearing sheet; and more particularly to single-channel position encoding for a scanning-head thermal-inkier printing machine and method that construct images from individual ink spots created on such a sheet, in a two-dimensional pixel array.
In the case of a machine for printing, the sheet is ordinarily of some printing medium such as paper, transparency stock, or other glossy medium.
Image-reading machines of the sort under discussion here are commonly called "scanners". These are not to be confused with printing machines that employ so-called "scanning" print heads--pens or matrix printing units that move back and forth across the printing medium to create image segments one line or swath at a time.
For purposes of this document, and particularly certain of the appended claims, the phrase "image-related device" is used to mean a printer or scanner.
2. Related Art
Printers which operate by using a scanning print head to mark on a print medium require, first, some provision for regularly keeping track of the head position and speed. Scanners as well, if operating with a movable reading head, have the analogous requirement.
In addition such systems require, at least at certain specific points, some information about the direction in which the head is moving. One reason for this second requirement is that it is a necessary condition for full satisfaction of the first requirement--in other words, for precise positioning information. This relationship will now be explained.
A well-known way to provide the position and speed information is by means of at least one electrooptical sensor that is moved in accordance with the print-head or reading-head movement, and that monitors a so-called "encoder scale". Because plural-sensor systems are more complex and expensive than singe-sensor systems, the present invention is limited to single-sensor systems; however, it is able to do the job of a two-sensor system.
In either a single- or a plural-sensor system, the scale is disposed in correspondence with positions (that is, the full range of positions) of the head across the image-bearing sheet. Such a scale generally takes one of two forms:
(1) a linear strip (often denominated "codestrip") extended--and usually tensioned--across a bed or channel that holds the image-bearing sheet, the strip being directly adjacent and parallel to the print-head or reading-head motion; and PA1 (2) a circular, hub-mounted scale read against--for example--the shaft of a motor that drives the print head. PA1 On one hand, if the system reverses while the encoder sensor is reading (or in other words, is "on") a graduation, then the same edge of the graduation is traversed twice: once while entering the graduation while traveling in one direction, and a second time while leaving the same graduation but traveling in the opposite direction. PA1 On the other hand, if the system reverses while the encoder sensor is between graduations, two opposite edges of a graduation are traversed in sequence just before the system reverses--and then the same two edges of the same graduation are traversed again (in opposite order) just after. PA1 "The problem presented by the lack of [direction] information in the encoder output signals is handled by superimposing the additional . . . sweep limit bands, in pre-determined spaced positions beyond the print limit bands . . . on the scale." (emphasis added) PA1 a first multiplicity of graduations defined along the encoder-scale substrate, and PA1 a second multiplicity of graduations, also defined along the substrate and interspersed among the first multiplicity of graduations over at least a distance approximately corresponding to the full transverse dimension of such a visual-image-bearing sheet. PA1 an encoder-scale substrate disposed in correspondence with positions of the carriage across the holding means, and PA1 some means for automatically sensing position of such a carriage along the support means. PA1 some means for automatically distinguishing between the graduations of the first multiplicity and those of the second multiplicity, and PA1 means for making positional determinations using the first multiplicity of graduations for a first purpose, and for making positional determinations using the second multiplicity of graduations for a second purpose. PA1 first means for using the first multiplicity of graduations for establishing position during carriage motion that starts from an edge of the holding means, and PA1 second means for using the second multiplicity of graduations for initializing position establishment during carriage motion that starts between the edges of the holding means. PA1 (a) automatically monitoring the first set of graduations during travel of the carriage across the medium; PA1 (b) automatically controlling the image transducers, based upon the automatic monitoring of the first set of graduations; PA1 (c) automatically maintaining an arrangement for preparing, during travel of the carriage across the medium, to use the positions of the second, distinctive set of graduations interspersed among the first set for reinitialization of counting of the first set of graduations at reversal if reversal is permitted when the carriage is only partway across the medium; PA1 (d) automatically determining whether image information permits reversal of the carriage when the carriage is only Partway across the medium; and PA1 (e) in event reversal partway across the medium is permitted, then: PA1 interrupting the automatic monitoring of the first set of graduations after movement of the carriage past a position corresponding to one of the second set of graduations, and PA1 discarding the count at or before passing of the carriage past the same position in the opposite direction. PA1 maintaining data that represents an image whose formation on the medium is desired, and PA1 during travel of the carriage across the medium, progressively applying the data to control the marking elements in forming the image on the medium; PA1 analyzing the data to be used in each pass of the carriage across the medium, to determine whether (1) any part of the image in each pass is to be formed after said monitoring of the second set of graduations has finished detecting a final individual graduation of the second set in that pass and before the carriage finishes that pass, and (2) for each pass, any part of the image in the following pass, in the reverse direction, is to be formed before said monitoring of the second set of graduations will detect a first individual graduation of the second set in that following pass, and PA1 in event no part of the image in that pass and the following pass is to be so formed, then determining that reversal is permitted when the carriage is only partway across the medium. PA1 extrapolating a preceding sequence of graduations of the first set; and PA1 operating the image transducer in accordance with the extrapolation.
Every such system has some arrangement for initializing the counting of graduations, starting precisely at a well-defined edge of the image area. Counting then continues across that area within a controlled range of speed so that the automatic equipment can, in effect, lock onto the progressively changing position. Once initialized, the system can maintain this lock as long as movement continues in the same direction.
Such a system, however, if operating with a single sensor generally speaking produces substantially identical pulse trains during both directions of movement of the head along its path. Previous workers have recognized that the resulting directional ambiguity is undesirable under certain circumstances--in particular when the moving head reverses direction.
A primary difficulty with such ambiguity is that the system can lose track of position just at--or just before or after--the moment of reversal:
The optical sensing system cannot distinguish these two cases.
Thus for example if a graduation is sensed as a dark part of the encoder scale, in the first situation the sensor detects first a light-to-dark transition as the leading edge of the graduation is crossed, and then (after reversal) a dark-to-light transition as the same edge is crossed again. In the second situation too, as the same graduation is traversed twice going in opposite directions, the sensor detects a light-to-dark transition and then a dark-to-light transition as the graduation is passed just before reversal--and then again a light-to-dark and then dark-to-light pair of transitions just after reversal.
Each sequence of light-to-dark and then dark-to-light transition pairs, in this second case, is optically and electrically indistinguishable from the single transition pair in the first case. As a result, when the carriage slows, stops and then starts moving in the reverse direction the system is unable to establish whether the carriage is just leaving a graduation in which it already stopped and reversed, or just leaving a graduation in which it did not quite stop and is now about to stop and reverse--or possibly even just leaving a graduation which it entered immediately after stopping and reversing.
In this way the directional ambiguity in a single-sensor system, if not resolved, would be transformed into a positional ambiguity of at least one graduation. After many reversals a much larger positional ambiguity would accumulate.
It will be understood that for fine positional precision the graduations are spaced very closely, and accordingly each graduation must necessarily be very narrow. The stopping distance and precision of the marking or reading head--for rapid scanning such as called for by high throughput--is not readily made equal to a small fraction of the periodicity of these fine graduations.
Therefore the ambiguity cannot be easily resolved by design adjustment of the relative magnitudes of graduations vs. stopping precision. The general result, for a single-sensor system with no provision for eliminating the ambiguity, would be loss of accurate positional lock, upon reversal of the head.
One well-known way to resolve the ambiguity is to provide not just one but two sensors, both reading the same codestrip but mutually offset along the line of motion by a known distance. In particular it is known in such a so-called "dual-channel encoder" to offset the two sensors by one-quarter of the overall periodicity of the graduations on the encoder scale (or by that distance plus or minus an integral number of periodicities), resulting in two electrical pulse trains in quadrature.
The direction of print-head motion is then ascertainable automatically through comparison of the two pulse trains. Such systems work well but are objectionably expensive in that they require an additional sensor and associated electronics.
U.S. Pat. Nos. 4,786,803 and 4,789,874 of Majette et al. describe representative systems for avoiding these costs, while nevertheless resolving the ambiguity, in a single-channel encoder. Majette et al. accomplish these goals, in a scanning-head printer, by providing special features on the encoder scale--but not within the range of operation in which the print head produces markings on the print medium.
These special features define the edges of that range of operation, and define positions at which the print head should reverse direction, and also define a position at which the print head should be parked. The positions designated as defining the extrema of actual marking by the print head are called "print limits", and those designated for change of direction are called "sweep limits" and sometimes "turn-around points"--or just "turnarounds" (although the head ordinarily is not actually reoriented for the reverse motion).
These features which define the print limits, sweep limits, and parking location on the Majette et al. encoder scale are graduations similar to but distinguished from the position-and-velocity graduations. The limit and parking-location graduations are distinguished by being wider than the position-and-velocity graduations. As Majette et al. explain:
Operationally, in terms of firmware that controls movements of a print head, this system depends upon the presence of the print-limit band, at each edge of the print medium, to signal that the next wide band which is reached is a sweep-limit band and so should be used as a reversal or turnaround point. In principle the system can be made to either (1) carry the sensor just past the sweep limit band and then return back through it, or (2) carry the sensor into the sweep limit band and reverse while the sensor is within that band.
Then, after reversal, the associated print limit band is used again for signaling the apparatus to initialize the counting of fine graduations--for the imminent pass across the print medium.
This system is entirely satisfactory for print heads or image-reading heads that operate all the way across the image-bearing sheet before reversing direction. Not all scanning-head systems, however, operate in that fashion.
In particular the limit-band system is not usable in a so-called "logic seeking" system. In such a system the moving head, to save time, reverses when the carriage is only partway across the image-bearing sheet--if the desired image or the image being read has no additional elements to be printed or read in the swath or line where the head is working.
In such a system, in effect, sweep limits (turnaround points) for a given pass of the head may be within rather than beyond the fixed print or read limits, at both ends of each pass. Even at the nominal starting side for each full scan, the head advantageously can be reversed before reaching the nominal image edge if there is no detail to be printed or read near that starting edge.
In some systems the head actually prints or reads while traveling in each of the two directions. In others the head only returns quickly in one of the two directions to a starting position for the next scan.
Logic-seeking is usable in both these types of mechanisms, as well as at both ends of each pass. In particular, even a head that prints or reads unidirectionally must move bidirectionally, and therefore can advantageously be made to reverse before reaching the scan-starting edge, where appropriate.
Such logic-seeking systems necessarily incorporate some means for automatically determining whether image information permits reversal of the carriage when the carriage is only partway across the image-bearing sheet. If so, for a particular line or swath, then these means must also determine exactly where reversal should occur--and should provide in some way for maintaining or renewing position lock at the time of reversal.
In the case of a printing machine, such means most typically preread and evaluate the data to be printed, before beginning each line or swath.
For a scanner, a like function perhaps may be performed by a preview head that only inspects the image quickly to find its extrema and so determine reading limits. Perhaps for special purposes the equivalent function might be accomplished by preprinting an image delimiter element or code at each end of each line or swath of an image that is to be scanned later--and then reversing each reading pass after reading the preprinted delimiter element or code.
In any event, once it has been determined whether the head carriage should reverse when only partway across the image-bearing sheet, reversal can be effectuated accurately--without losing position lock--by a dual-channel encoder, but as explained earlier not by a single-channel encoder such as introduced in the '803 and '874 patents.
Therefore the prior art leaves unsolved the problem of maintaining accurate positional lock at reversals of a marking or reading head, in a logic-seeking system with a single-channel encoder. That is, prior art in image-related devices of the scanning-head type fails to teach any way to enjoy the throughput benefits of logic-seeking in combination with the economic benefits of a single-channel encoder--and accurate positioning.
Accordingly, important aspects of the technology which is used in the field of the invention are amenable to useful refinement.